Every year at some point during the season we seem to hit a rainy spell where it seems to rain at some point every day and we have showers widely scattered across the state. It started raining last Wednesday and we have had some rain everyday since then and it is currently raining in Stoneville right now (Monday morning). With that, the questions start about how long you need to spray before a rain to get acceptable control.
This is a very difficult question to answer and there is not a lot of research data out there on rainfastness. The reason it is difficult to give a good answer is because it is a very complex situation that can be impacted by several different factors besides just time after the spray. The data that we do have are under specific conditions and those conditions will likely not be the same in every situation. Some factors that can impact the performance of an insecticide during rainy weather include (but are not limited to):
Amount of rainfall – This is fairly intuitive. Obviously 1 inch of rain will have more of an impact than a half an inch of rain, and 2 inches of rain will likely have a greater impact than 1 inch of rain. However, any amount of rain can have a negative impact on some insecticides.
Intensity of rainfall – This is an area we don’t know a lot about. You would expect that a half inch of rain that falls over a 20 minute period would have more of an effect than a half inch of rain that falls over a 2 hour period. In other words, a hard driving rain is probably going to be worse than a slow steady rain.
Activity of the Insecticide – Activity refers to whether or not the insecticide being used has systemic activity (moves into the plant) or contact activity. Systemic insecticides tend to be a little more rainfast than most contact insecticides because they can not be washed off once they are in the plant. In general, most of these will move into the plant within about 2-4 hours. Some contact insecticides may not be very rainfast, but it may not matter. This can be very complex depending on the specific crop and targeted pest (see below)
Target Insect – Where the insect feeds can be very important, especially with contact insecticides. For insects that feed enclosed within plant structures (such as bollworms in cotton or soybeans) or mobile insects that are migrating into the field (tarnished plant bugs), residual control is very important. In those situations, a contact insecticide like a pyrethroid or organophosphate may not do a very good job because the rain will wash it off of the foliage before they are moving around in the crop. In other situations where the insect is exposed (bollworms in small beans or grain sorghum, or rice water weevil in rice), a pyrethroid may be the best choice because those insects will come into contact with a lethal dose almost immediately and will be controlled. If the rain washes it off after that point, it may not really matter.
For most insecticides, effective control can be obtained if the spray goes out about 2 to 4 hours before the rain. One major exception to this is Orthene or any of the other acephate based products. Acephate is by far the least effective insecticides to use when rain is expected, in fact generally we like to see 8-12 hours after acephate, but longer the better.
Adjuvants can help with rainfastness. We did some work with adjuvants a couple of years ago and found that any good adjuvant can improve control with most insecticides. However, an adjuvant can’t fix a situation where an insecticide like acephate has poor rainfastness.
As far as the thrips materials go, Bidrin and Dimethoate would likely be the best option if rain is immanent. We really have no available data on Radiant and rainfastness but my guess is, based on the formulation, likely 4 to 6 hours. And as previously mentioned, acephate needs the most time.
One final thought. Remember that control of some pest can vary greatly with different nozzle types. Coarse droplet low drift tips reduce efficacy on pest like thrips. Hollow Cone or Flat Fan tips with high P.S.I will greatly improve control. This will become magnified even more when frequent rains are occurring. When mixing thrips products with herbicides it is understandable (and wise) to choose low drift tips but it will likely have some impact on control.